A variety of types of conventional acoustic transducers are known. Acoustic transducers are used to convert electrical energy to acoustical energy, and vice-versa. An acoustic projector, a type of acoustic transducer, in operation, is used in one transduction direction, to convert electrical energy to acoustic energy. However, though usually not used in the opposite direction, an acoustic projector also does operate in the opposite direction, to convert acoustic energy into electrical energy.
Some conventional acoustic transducers have limitations. For example, many types of conventional acoustic transducers are not capable of producing large amounts of acoustic power at low frequencies, for example, on the order of two kHz or less, and, in particular, under 400 Hz. Similarly, many types of conventional acoustic transducers are unable to operate over a wide bandwidth at the low frequencies.
Physical limitations can complicate solving such deficiencies. For example, an ability to withstand high stresses is important for deep depth, high water pressure, survival and operation, as well as for the ability to produce high acoustic power levels. An ability to withstand high stresses can result in an inability to operate at the above-described low frequencies.
Known types of acoustic projectors that can operate as acoustic projectors at low frequencies include flextensional transducers, inverse flextensional transducers, bender disc transducers, wall-driven oval transducers (also know as “WALDOs”), and slotted cylinder acoustic transducers.
Slotted cylinder acoustic projectors can be characterized by various parameters, including, but not limited to, a center operating frequency, a bandwidth (associated with a mechanical Q), and an efficiency corresponding to a ratio of acoustic power output to electrical power input.
Referring to FIG. 1, one type of conventional slotted cylinder acoustic transducer 10, here shown as a cross section, includes a cylindrical housing shell 12 having an inner surface 12a, an outer surface 12b, and a central major axis of curvature 14 (perpendicular to the page). The cylindrical housing shell 12 has a tapered thickness 12c between the inner surface 12a and the outer surface 12b, wherein the thickness 12c is taken in a direction perpendicular to the central major axis of curvature 14. The cylindrical housing shell 12 also has a slot 16 through the housing shell 14 extending in a direction parallel to the central major axis of curvature 14, forming a gap 18 in the cylindrical housing shell 12. The thickness 12c of the cylindrical housing shell 12 is greatest at a position opposite the slot 16 and smallest at positions proximate to the slot 16.
The slotted cylinder acoustic transducer 10 also includes a plurality of ceramic elements 20, of which a ceramic element 20a is but one example, having different solid shapes, and each having a respective central major axis (e.g., 32, perpendicular to the page). It will be recognized that the shapes of the ceramic elements 20 symmetrically on either side of and equidistant from an axis 22 can be the same. However, it will be recognized that there are approximately as many different shapes of ceramic elements 20 as half of a total number of ceramic elements 20.
The slotted cylinder acoustic transducer 10 also includes a plurality of electrodes 24, of which an electrode 24a is but one example, having different planar shapes. Each electrode 24 is disposed between two adjacent ceramic elements 20. It will be recognized that the shapes of the electrodes 24 symmetrically on either side of and equidistant from an axis 22 can be the same. However, it will be recognized that there are approximately as many different shapes of electrodes 24 as half of a total number of electrodes.
The plurality of ceramic elements 20 and the plurality of electrodes 24 are interposed in a ceramic stack assembly 26, one of the electrodes 24 between each two adjacent ceramic elements 20. The ceramic stack assembly 26 has an inner surface 26a, an outer surface 26b, a central major axis of curvature 14a (perpendicular to the page) parallel to the central major axis of curvature 14 of the cylindrical housing shell, and tapered thickness 26c between the inner surface 26a and the outer surface 26b, wherein the thickness 26c is in a direction perpendicular to the central major axis of curvature 14a of the ceramic stack assembly 26. The central major axis of curvature 14a of the ceramic stack assembly 26 can be at the same position as the central major axis of curvature 14 of the cylindrical housing shell 26, or it can be at a different position as shown.
A shape of the outer surface 26b of the ceramic stack assembly 26 matches a shape of the inner surface 12a of the cylindrical housing shell 12. The outer surface 26b of the ceramic stack assembly 26 is disposed proximate to the inner surface 12a of the cylindrical housing assembly 12.
The slotted cylinder acoustic transducer 10 can further include first and second tapered inserts 28, 30, respectively. The tapered inserts 28, 30 have inner surfaces 28a, 30a, respectively, outer surfaces 28b, 30b, respectively, and central major axes 34, 36, respectively (perpendicular to the page). Shapes of the outer surfaces 28b, 30b of the first and second tapered inserts 28, 30, respectively, match the shape of the inner surface 12a of the cylindrical housing shell 12. The outer surfaces 28b, 30b of the first and second tapered inserts 28, 30, respectively, are disposed proximate to the inner surface 12a of the cylindrical housing shell 12. The first tapered insert 28 is disposed proximate a first end of the ceramic stack assembly 26 and the second tapered insert 30 is disposed proximate to a second end of the ceramic stack assembly 26.
The slotted cylinder acoustic transducer 10 can include end caps (not shown) and an outer boot (not shown) so as to be sealed from the water.
It will be appreciated that having so many different ceramic elements 20 with different shapes and so many electrodes 24 with different shapes tends to make the acoustic transducer 10 expensive.
As described above, the acoustic transducer 10 can be characterized by various parameters, including, but not limited to, a center operating frequency, a bandwidth (associated with a mechanical Q), and an efficiency corresponding to a ratio of acoustic power output to electrical power input.
It will be understood that the center operating frequency of the slotted cylinder acoustic transducer 10 is related to a number of parameters, including, but not limited to, a density, stiffness, and modulus of elasticity of the cylindrical housing shell 12, a density, stiffness, and modulus of elasticity of the ceramic stack assembly 26, and a density, stiffness, and modulus of elasticity of the first and second tapered inserts 28, 30, respectively. As is known, stiffness is related to the modulus of elasticity of a material of an object, a shape of the object, and boundary conditions experienced by the object. A higher modulus of elasticity generally results in a higher operating frequency, and a higher density generally results in a lower operating frequency.
In order to design the slotted cylinder acoustic transducer 10 to achieve a particular center operating frequency, properties of the components, or of the entire slotted cylinder acoustic transducer 10, can be modeled using a finite-element computer model. The finite element model should include a so-called “radiation loading” of the water around the slotted cylinder acoustic transducer 10, which is related to an acoustic impedance. Finite element models can predict both static stresses and dynamic stresses upon elements of the transducer 10. Finite element models can also predict dynamic behavior of the slotted cylinder acoustic transducer 10.
It will be understood that the bandwidth of the slotted cylinder acoustic transducer 10 is related to a ratio of largest and smallest thicknesses 12c of the tapered cylindrical housing shell 12 and a ratio of largest and smallest thicknesses 26c of the tapered ceramic stack assembly 26 in combination with the tapered inserts 28, 30. In addition, it is known that bandwidth of the slotted cylinder acoustic transducer 10 generally increases when a length of the cylindrical housing shell 12 of the slotted cylinder acoustic transducer 10 is increased relative its outer diameter.
An efficiency of the slotted cylinder acoustic transducer 10 is related to a variety of factors, including, but not limited to, characteristics of a rubber boot surrounding the slotted cylinder acoustic transducer, piezoelectric efficiency of the piezoelectric ceramics 20 (related to a dielectric loss resulting in heating), and losses in bondings associated with the ceramic elements 60.